In recent years, office buildings and houses have introduced distributed generations, such as solar power systems and fuel cells. The surplus electric power remaining after subtracting the power consumed by loads from the power generated by the distributed generations flows back to a distribution system, and is sold to electric power companies.
In order to achieve greater proliferation of the distributed generations, the Japanese government set, as a national policy, the target amounts of power generated from the newly introduced solar power system to 2.8 million kW in 2020, and 5.3 million kW in 2030. However, if a large number of distributed generations are interconnected to a distribution system and the surplus electric power flows back to the distribution system, it has been pointed out that the voltage may rise at points of common coupling and the voltage may be out of the appropriate voltage values for low voltage consumers (101±6V, 202±20V).
In order to prevent the voltage for the low voltage consumers from being out of the appropriate voltage values, the guideline established by the Ministry of Economy, Trade and Industry proposes a technique capable of automatically regulating a voltage by supplying the leading reactive power from the distributed generations or suppressing the active power to be output to the distributed generations, when it is likely that the voltage for the low voltage consumers is out of the appropriate voltage values due to the reverse power flow from the distributed generations (NPL 1).
However, when the larger number of distributed generations are introduced, under the guideline, the voltage tends to rise higher at a connection point closer to the end of the power distribution line and more distant from the distribution substation and the transformer. As a result, as a distributed generation is closer to the end of the power distribution line, the supplied value of the leading reactive power or the suppression value of the active power further increases (NPL 2).
In order to cope with the problems, for example, PTL 1 discloses the technique of (i) holding a voltage-controlled start threshold that is set to control the voltage to fall below a voltage upper limit value and that is set to a higher value as the interconnection position to the power distribution line is more distant from the starting position of the power distribution line, and (ii) controlling a voltage by supplying the leading reactive power when the voltage at a connection point exceeds the voltage-controlled start threshold. Accordingly, the surplus electric power can be sold to electric power companies almost in an equitable manner, regardless of the setting positions of the distributed generations.
PTL 2 discloses the technique of searching for a voltage deviation portion and controlling voltages at the distributed generations by collecting, using a monitoring control server, state data of a distribution system and operating data of the distributed generations and estimating the distribution of voltages of the distribution system. The technique allows suppressing variations between the distributed generations when the output power from the distributed generations is to be adjusted.